Walsh codes are used to differentiate channels in a code division multiple access (CDMA) cellular wireless communication system. Some of the Walsh codes are allocated to dedicated channels, such as a fundamental channel (FCH) and a supplemental channel (SCH), or reserved for overhead channels. In a CDMA cellular wireless communication system that supports packet data services, such as those defined in Revision C (and above) of the IS-2000 standard, a subset of available Walsh codes is allocated for the transmission of data packets on a forward data packet channel (F-PDCH).
Usually one of several Walsh tables is selected through upper layer signaling, and Walsh codes are selected sequentially through the selected table using a Walsh mask. The Walsh mask is sent through a separate channel (i.e., a forward packet data control channel or F-PDCCH). The Walsh mask indicates the entries of the selected table to be skipped (masked) when selecting Walsh codes.
There is a certain probability for a mobile station to falsely decode a F-PDCCH frame and obtain a false Walsh mask. The probability can be as high as 1.5×10−5. When a mobile station obtains a false Walsh mask, all subsequent decoding of the forward packet data channel (F-PDCH) will be wrong, until an update of the Walsh mask is sent by a base station and the mobile station correctly receives the updated Walsh mask.
It would be desirable to have a Walsh mask test in code division multiple access (CDMA) cellular wireless communication systems with packet data service.